Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Chanchal Sarbajna ¹, P. Krishnamurthy ¹, A. K. Paul ¹, D. C. Banerjee ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Begumpet, Hyderabad - 500 016, IN

Abstract

Detailed mineralogical and chemical studies by EPMA on the Nb-Ta minerals from the rare metal pegmatites of southern Karnataka have revealed wide compositional variations, and the presence of some new minerals like tapiolite and microlite, hitherto unreported from these pegmatites. Back scattered electron (BSE) images have indicated complex zoning patterns such as oscillatory, patchy and/or their combinations, associated with replacement of columbite-tantalite by microlite, manifested by the variations of Nb₂O₅, Ta₂O₅, FeO and MnO. Such features, although known from a few rare meral pegmatites of the world, are being recorded here for the first time in India. Oscillatory zones in columbite-tantalites are manifested as alternating dark and light grey coloured bands, of which darker bands are Nb-rich (30-300 μM thick) and lighter bands are Ta-rich (125-450 μm thick). The wider zones may, in turn, consist of a group of very fine sub-zones (1-50 μm) of slightly varying composition: Patchy zoned crystals exhibit corroded remnants of early formed columbite-tantalite (with 39.73-44.02% Ta₂O₅), surrounded by later formed zones which are enriched in Ta₂O₅ (up to 50.99 %) in columbite-tantalite leading to the formation of microlite containing up to 73.4% Ta₂O₅. The zoning of the columbite-tantalites has been attributed to periodic changes in the composition of the major components such as Nb, Ta, Fe and Mn in the pegmatite fluid system, apparently influenced by the late stage fractionation of volatiles. This ultimately generated fluids rich in Ta and Na, resulting in resorption and replacement with patchy zoning of early formed, zoned columbite-tantalites.

Keywords

Columbite-Tantalite, Rare Metal Pegmatite, Zoning, Ore Mineralogy, Southern Karnataka.

Full Text

     

Chanchal Sarbajna ¹, R. P. Sinha ¹, P. Krishnamurthy ¹, K. V. G. Krishna ², R. Viswanathan ¹, D. C. Banerjee ¹

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Begumpet, Hyderabad - 500 016, IN
2 Atomic Minerals Directorate for Exploration and Research, Nagarbhavi, Bangalore - 560072, IN

Abstract

White coloured alkali beryl from the pegmatites of Mariagalla-Allapatna in Mandya district, Karnataka contains distinctly higher alkali elements (Na, Li, Rb, Ca) and relatively low beryllium oxide (c. 11 % BeO) compared to normal beryl. Beryl occurs mostly between graphic intermediate zone and quartz core in the complexly zoned, spodumene-bearing Li-Cs-Ta (LCT) type pegmatites in this area. The average value (n = 7) for alkali beryl from this area are Na₂O 1.26%, Li₂O 0.6%, Cs₂O, 0.74%, K₂O 0.07 and Rb₂O 0.08%. Cs content in some of the beryls are higher than Na and hence such beryls may be termed as a new Na-Li-Cs type X-ray powder diffraction data for the beryl (a_o = 9.2765-9.339 Å, c_o = 9.2079-9.2487 Å) reveal differences in crystal structure with respect to normal beryls from Royalstone, Massachusetts, USA (a_o = 9.215Å, c_o = 9.192Å) and compares well with the reported alkali beryl from Arizona, USA (a_o = 9.30Å, c_o = 9.20Å) Their refractive index (N_ω) ranges from 1.588 to 1.592 and the specific gravity varies from 2.74 to 2.79.

Keywords

Economic Geology, Geochemistry, Mineralogy, Rare Metal Pegmatites, Beryl, Marlagalla, Karnataka.

Full Text

     

D. C. Banerjee ¹, N. Ranganath ¹, P. B. Maithani ¹, K. M. V. Jayaram ¹

Affiliations
1 Atomic Minerals Division, Begumpet, Hyderabad 500016, IN

Abstract

Several pegmatites containing commercially viable sources of columbite-tantalite have been located in the Holenarsipur, Kdshnarajapet and Nagamangala schist belts. The pegmatites in Holenarsipur schist belt near Hassan, contain columbite-tantalite, rich in both niobium and tantalum and carry appreciable amounts of beryl. The Nagamangala schist belt is particularly rich in tantalite-bearing (Ta₂O₅ 50-70%) pegmatites.

Analysis of feldspars from the three different belts indicates that they are potassic in the Nagamangala belt while they are sodic in the case of two other belts. Trace element analysis of muscovite indicates an average of 42, 242, 107, 325 ppm of Be, Nb, Sn and Li in Nagamangala belt as against 55, 114, 127 and 329 ppm respectively in Holenarsipur belt. Presence of considerable amount of bismutite in Mundur pegmatite and traces of spodumene and pollucite in Nagamangala belt have been confirmed.

Full Text

     

D. C. Banerjee ¹, K. V. G. Krishna ¹, G. V. G. K. Murthy ¹, S. K. Srivastava ¹, R. P. Sinha ¹

Affiliations
1 Atomic Minerals Division, Dept. of Atomic Energy, Begumpet, Hyderabad-500016, IN

Abstract

Spodumene bearing rare metal pegmatites are reported mostly from Karnataka in contrast to lepidolite and amblygonite bearing pegmatites described from major mica-fields of India. Tantalite (Ta₂O₅ 50-70%) rich pegmatites intruding the Sargur metasediments of Nagamangala schist belt near Marlagalla- Allapatna are also rich in spodumene (10-15% by vol.) as well as beryl (10.6-11.4% BeO). Light green spodumene (Li₂O 6.55-7.35%) occurs randomly as prismatic crystals a few cm to 30 cm in length. Under the microscope, symplectic intergrowth between spodumene and quartz along the contact with albite and replacement of spodumene by quartz and muscovite are noticed. The 'd' spacings and intensities obtained from the X-ray diffraction patterns match well with the published values for Alphaspodumene from Pala San Diego County, California, U.S.A. Some variations observed in the intensities ofinterplanar spacings could be due to preferred orientations exhibited by the mineral. When heated to 1000° C for 3 hours, the Alpha-spodumene transformed to Beta- spodumene.

Chemically these spodumenes are "high iron non-chromian" types (FeO+Fe₂O₃ 1.17 to 1.29%, Cr47- 87 ppm). While Mn content is high (1237-1826 ppm), other trace elements such as V, Cu, Co, Pb and Be are less than 10 ppm, Cs 39-98 ppm and Sn 19-28 ppm. Sn values are considerably low as compared to specimens reported from elsewhere, possibly because these pagmatites are devoid of cassiterite. Mineralogical assemeblage ofspodumene-tnntalite-pollucite in these pegmatites places them in "LCT (Lithium-Caesium-Tantalum) family" of rare element pegmatites as per classification of Cerny. Petromineralogical and geochronological studies of Allapatna granites have been undertaken which will throw furthur light on the geochemistry and paragenesis of these pagmatites. Few hundred tonnes of spodumene are expected to be recovered commercially from shallow depths alone along with tantalites and beryl.

Keywords

Spodumene, Symplectic Texture, Pegmatites, Petrology, Kamataka.

Full Text

     

S. D. Rai ¹, D. C. Banerjee ¹

Affiliations
1 Atomic Mineral Division, Begumpet, Hyderabad - 500 016, IN

Abstract

Several inland placers containing commercially viable resources of xenotime have been described from the Chhotanagpur granite gneiss terrain of Gumla district, Bihar and Raigarh district, M.P. Recent surveys in the adjoining western sector of Surguja district, have confirmed xenotime as a significant minor constituent (3-5 %) in the heavy mineral assemblages (1-4 %) of the various tributaries , of Mahan river.

The assemblage is typical characteristic of a granite migmatitic provenance. Geomorphologically, few of these occurrences are developed over limited extents as poorly sorted sediments in low order, less mature streams having steeper slope and smaller depth with an insignificant flood plain development. Mineralogically, the heavy mineral assemblages consist of ilmenite ± garnet (10-40 Wt%) with monazite (40-70 Wt%), zircon (10-20 Wt%) and magnetite (traces to 5 Wt%). A few of these placers are being investigated as potential sources for Yttrium as their heavy mineral suites, quantitatively, have a higher total content of xenotime and nronazite for upgradation to a marketable product acceptable to the chemical industry.

Keywords

Xenotime, Stream Placers, Economic Geology, Mahan River Basin, Madhya Pradesh.

Full Text

     

K. Umamaheswar ¹, Himadri Basu ¹, J. K. Patnaik ¹, Mir Azam Ali ¹, D. C. Banerjee ²

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Bangalore-560 072, IN
2 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Hyderabad-500 016, IN

Abstract

Significant uranium occurrences are discovered recently for the first time in the Gulcheru Quartzite of Papaghni Group of Cuddapah Supergroup near Gandi, east of Papaghni river, Cuddapah District, A.P. Mineralisation is associated with an E-W fault. Friable whitish to light pink quartzite is the main host rock of uranium. Mineralisation occurs in the form of lensoidal bodies having dimensions of 20 - 200 m x 2 - 10 m in an E-W fault scarp section, over 2 km in length. Dip section is also mineralised. Samples have revealed anomalous concentrations of uranium, up to 1.441% U₃O₈ with negligible thorium. Concentration of Ni (<25 to 1033 ppm), Mo (<25 to 643 ppm), Pb (33 to 1515 ppm) and Cu (<25 to 170 ppm) points to the economic potentiality of the area. Petrological studies have identified uranium minerals such as pitchblende, uraninite and coffinite as fracture fillings and disseminations in the chlorite-sericite matrix of quartzite. The associated ore minerals are chalcopyrite, arsenopyrite, pyrite and limonite. These findings have important implications in the selection of new exploration targets for uranium in the basal beds of Cuddapah Supergroup unconformably overlying the fertile granitoid basement in the Eastern Dharwar Craton occurring in the southwestern part of Cuddapah basin.

Keywords

Uranium Mineralisation, Gulcheru Quartzite, Cuddapah Basin, Andhra Pradesh, Dharwar Craton.

Full Text